The search for blood substitutes has been prompted by many serious drawbacks to the use of donor blood. One of the most serious problems relates to the fact that an adequate supply of compatible donor blood may not be available at the time and place where it is needed. This would be a particularly serious problem during periods of disaster or war when blood is most needed. The incompatibility of different blood types and the relatively short storage life of whole blood limits the practicability of collecting and storing large amounts of whole blood. According to some estimates, as much as 30% of human blood collected is not reinfused into human patients. Although red blood cells have a relatively short storage life which contributes to a large amount of wasted blood, the hemoglobin contained in the red cells appears to be unaffected if stored under the proper conditions. However, the red cell membranes tend to degrade in storage. Further, the transmission of disease, especially hepatitis, is a problem that makes physicians hesitant to transfuse whole blood or plasma into patients under conditions that are not life-threatening to the patient. Thus, there is a definite need for effective blood substitutes.
The primary function of a plasma expander or blood substitute is to maintain an adequate circulation volume of a solution that is nontoxic to the human body and which will transport oxygen throughout the body to sustain life until the body can remanufacture a supply of natural blood. Artificial blood substitutes have received considerable attention during the last decade. The development of fluorochemical emulsions that transport oxygen and carbon dioxide in the blood stream offered hope of a viable blood substitute. Unfortunately, however, a number of problems have limited the use of fluorochemical emulsions in human patients.
Aqueous hemoglobin solution is the main constituent of red blood cells and is the blood's oxygen carrier. It has also been demonstrated to be safe when used in human patients. However, its intravascular persistance and oxygen-release characteristics have proven to be inadequate. Free hemoglobin completely disappears from the blood stream in less than about eight hours and its affinity for oxygen is much greater than that of an equivalent amount of hemoglobin encapsulated within the natural red-blood-cell membrane. This greater affinity makes liberation of oxygen to the tissues much more difficult. It is the red-blood-cell membrane that contains the antigenic material which causes problems when mismatching of donor and recipient occurs. Problems of intravascular coagulation and renal damage have been demonstrated to be caused by the stroma present in such membranes and not the hemoglobin molecule itself. Thus, the use of stroma-free hemoglobin solution (SFHS) as a blood substitute offers many potential advantages regarding immunology, storage and bio-compatibility. Also, since free hemoglobin is not toxic to the kidney, SFHS seems an ideal starting material for a blood substitute.
A number of attempts have been made using standard encapsulated techniques employing various polymeric materials such as celluloses, polystyrenes and polyamides to create artificial red cells by encapsulating hemoglobin therein. However, these standard encapsulation techniques have produced red cells non-biodegradable by ordinary metabolism. Alternatively, U.S. Pat. Nos. 4,001,401; 4,053,590; and 4,061,736 disclose blood substitutes and plasma expanders comprising polymerized, cross-linked, stroma-free hemoglobin in either the oxyhemoglobin or deoxyhemoglobin form, said polymers having a molecular weight ranging from about 64,000 to 1,000,000. These blood substitutes are prepared by cross-linking stroma-free hemoglobin in bulk solution with a suitable cross-linking agent that is at least bifunctional in nature. However, the hemoglobin polymers are not artificial red cells, because they are merely polymeric molecules as such and do not consist of a membrane encapsulating a fluid phase which can reversibly combine with oxygen. In another attempt at creating artificial red cells, Miller et al in U.S. Pat. No. 4,133,874 disclose forming artificial red cells by encapsulating hemoglobin in liquid lipid materials comprising phospholipids, and optionally cholesterol, to form cells typically ranging from 0.1 to 10 microns in their greatest dimension. The lipid material is said to form a continuous membrane around the hemoglobin solution. However, phospholipids have a tendency to release their contents into other cells in the body. Kitajima et al in U.S. Pat. No. 3,879,510 show reinforcing the naturally-occurring membrane around red blood cells by reacting the membrane with an isocyanate such as toluene diisocyanate. In reinforcing the naturally occurring membrane, red blood cells are dispersed in a suitable isotonic or hypertonic saline solution to which is added an oil-in-water emulsion of liquid polyisocyanate which reacts with the membranes, thereby reinforcing same. However, it is the red blood cell membranes which contain materials that cause short storage life, and incompatibility problems between different blood types.
Thus, there is a need for artificial red cells comprising an encapsulated, stroma-free hemoglobin solution of a suitable size, strength and flexibility to permit same to be used effectively as blood substitutes.